Minimum protection upstream of a 100A fuse? 1

[AusLee]

Hi,

A 100A "High Rupture Capacity" fuse is supplying a tariff meter.

The curves of the 100A fuse and an upstream 160A circuit breaker set at 140A, when shown with tolerance/band, are touching at the long term delay section.

The curves of the 100A fuse and the 140A CB, when shown as centreline only, do not touch (fuse completely under circuit breaker).

The question is: what is the minimum circuit breaker required upstream of this fuse to ensure complete protection, and is the centreline trick legit?

Thanks.

 

[jghrist]

"ensure complete protection" is the wrong term. You want to ensure complete coordination. The smaller CB will protect against faults. It may trip unnecessarily for a fault downstream from the 100A fuse, however. You will have to look at the next size higher CB curves to see if it coordinates for the full range of possible fault currents downstream of the 100A fuse. If you only coordinate with the centerlines, then if the fuse and/or the CB don't operate at the average speed, you may not get coordination.
In some cases, it may not be practical to ensure complete coordination and you may decide to live with the possibility of the CB tripping unnecessarily.

 

[AusLee]

I'm getting an error when I try a C50 MCB downstream of the 100A HRC fuse. The gap is too large: from 50A in the MCB to 100A in the fuse, yet still I get an error and says either the MCB shall be smaller, like 40A or the fuse shall be 125A.

Is this normal? C50 MCB downstream of a 100A fuse and still not ok?

 

[ScottyUK]

It's a known problem - the 1.6x rule-of-thumb doesn't work when mixing fuses and breakers. The problem is the shape of the curves. The 100A fuse has a steep time-current curve, while the breaker has a more gentle curve and a pronounced knee where the thermal and magnetic characteristics meet. Coordination between fuses and MCBs is always awkward if the fuse is the upstream element - generally fuses discriminate with fuses, and downstream fuses discriminate with upstream breakers.

You will probably find that a Type B MCB will be fine if your load doesn't require the inrush capability of the Type C.

 

[healyx]

Hi Auslee,

In my opinion you shouldn't use the centreline only method. You should be checking tolerance bands. It doesn't surprise me that a 100A fuse and 140A breaker upstream aren't discriminating.

As for the MCB under the fuse I'd be really surprised if a 50A MCB discriminated with a 100A fuse upstream. It is not just about checking the time current curves, you need to check manufacturer tables to see what happens in the instantaneous region. MCBs under a fuse tend to have pretty low discrimination values. The problem is that fuses start current limiting at a relatively low current. A 100A BS88 fuse will start current limiting at about 1.3kA, so unless your prospective fault level is below this, you will not be able to achieve full range discrimination regardless of what MCB rating you use under it. It is a trade-off.

You might want to check AS3000. They have some ratios for upstream to downstream ratings that they deem to be OK for discrimination (not that I agree with them). I'm not sure they cover fuse/cb situation though....that got put in the too hard basket.

 

[healyx]

To clarify my last post, the 1.3kA I quoted for discrimination limit is of course subject to whether the lower circuit breaker can clear the fault before the fuse is exposed to enough energy to hit pre-arc. It is a rough guide. The combination needs to be tested - using curves aren't enough - hence why manufacturers publish tables for this.